Xylem

The epidermis of the roots is the outer layer of cells, consisting of flattened cells and root hair cells, which absorb water and ions. The xylem are long tubes made up of dead cells, and their function is to transport water and minerals up the plant. 

The structure of the xylem consists of:

• Narrow tubes made up of dead cells to minimize resistance to water movement.

• Strong walls made of lignin to prevent the xylem from collapsing, which also provides structural support around the xylem vessel.

The Cohesion-Tension Theory

Water evaporates from leaf cells and diffuses out of the stomata in a process called transpiration. 

Loss of water from a leaf

This creates a negative pressure (or tension), pulling water upward through the xylem.

• Cohesion refers to the attraction between water molecules, allowing them to ‘stick’ together due to hydrogen bonds. The cohesion of water molecules transmits the pulling force down the xylem to the roots.

• Adhesion refers to the attraction between water molecules and the sides of the xylem, due to hydrogen bonding with cellulose in the plant cell walls.

• The adhesion of water to the xylem helps to resist the force of gravity. This entire mechanism of water movement is known as the cohesion-tension theory.

The movement of water through xylem vessels.

Factors Affecting The Rate Of Transpiration

Various factors influence the rate of transpiration, such as wind speed, temperature, humidity, and light intensity:

• Windy conditions increase the concentration gradient of water vapor between the leaf and the air, resulting in a faster rate of transpiration.

• Higher temperatures give water molecules more kinetic energy, speeding up evaporation and diffusion.

• At low humidity, the concentration gradient of water molecules between the leaf and the atmosphere is greater, so diffusion occurs more quickly.

• In bright light, stomata open to allow the uptake of carbon dioxide for photosynthesis, causing more water to be lost through transpiration.

To measure the rate of transpiration, a potometer is used, which tracks the movement of an air bubble through a capillary tube against a scale.

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